The present invention relates to a method for regenerating anion exchange resins in bicarbonate form which are present in aqueous suspensions and are used for the removal of strong acid anions from untreated waters. In the process, a calcium compound in solid form and gaseous CO.sub.2 are employed simultaneously and the ion exchange resin is separated from the effluent (regenerate) after the regeneration process and reused.
In the prior art, anion exchange resins have been regenerated generally with alkali solutions, such as, for example, sodium hydroxide, sodium carbonate, or ammonium hydroxide solutions. In order to overcome the drawbacks of these processes during regeneration of weak base anion exchange resins, it has been proposed in German Offenlegungsschrift No. 2,530,677 to use a 1 to 5 weight percent calcium hydroxide suspension as the regeneration agent. The regeneration agent here is conducted from the bottom to the top through a granular ion exchange mass, is then expelled, and the ion exchange resin is then rinsed with water. The weak base anion exchange resin is thereby returned to its hydroxyl form and can be reused.
During the removal of strong acid anions, such as chloride, sulfate or nitrate ions, from untreated waters, it has been found that anion exchange resins in bicarbonate form give good results. For full desalination processes, such anion exchange resins in bircarbonate form are used in combination with cation exchange resins. U.S. Pat. No. 3,691,109 discloses such a process for regenerating the resins in a desalination system in which the combination of a weak acid cation exchange resin with a weak base anion exchange resin employed during the desalination of water is regenerated in a three-bed system when the ion exchange masses become exhausted.
In the process disclosed in U.S. Pat. No. 3,691,109, gaseous carbon dioxide is introduced into the cation exchange bed at a pressure of 0.35 MPa to 6.89 MPa (1 MPa=10 bar) in order to regenerate the weak acid cation exchange resin or to return its sodium form to the free acid form, respectively. The resulting effluent from this step, which contains sodium bicarbonate and free carbonic acid, is degassed, i.e. the carbon dioxide is removed, and the solution obtained in this way is used to regenerate the weakly basic anion exchange resin. The regenerated ion exchange resins are then charged in a countercurrent direction with the water to be desalted. Thus, after regeneration, the anions are first removed from the untreated water in bed 3 which contains the weak base anion exchange resin. Then, in bed 2, calcium and magnesium ions are precipitated as Mg(OH).sub.2 and CaCO.sub.3 with the aid of an added calcium hydroxide suspension and only thereafter, in bed 1, are monovalent cations eliminated by the weak acid cation exchange resin. When the ion exchange resins are exhausted, the cycle begins anew with the regeneration step.
It is disclosed in U.S. Pat. No. 3,691,109 that water which is saturated with carbon dioxide at a carbon dioxide pressure of 0.5 to 1.0 MPa has a pH of about 3.3, at which pH the weak acid cation exchange resin has a negligible capacity for monovalent and bivalent ions. Particularly preferred, however, are carbon dioxide pressures between 0.7 and 2.0 MPa because under these conditions the process is claimed to operate most efficiently.
The process disclosed in U.S. Pat. No. 3,691,109 is directed toward a full desalination--regeneration of the ion exchange masses employed there, and particularly is concerned with the regeneration of a combination of weak acid cation exchange resins and weak base anion exchange resins. The process thus cannot be used universally. Further, the process can be performed only in a relatively complicated system and requires costly chemicals for the removal of the calcium and magnesium ions, in particular, Ca(OH).sub.2 or CaO.
An article entitled Desalination of Brackish Water by Ion Exchange, by A. C. Epstein and M. B. Yeligar, appearing in "Ion Exchange and Membranes", Vol I, pp 159-170 (1973), provides a review of four ion exchange systems for the desalination of water. In the so-called Desal Process, which employs a three-bed system, a weak base anion exchange resin in bicarbonate form is used in the first column, a weak acid cation exchange resin in the free acid form is used in the second column and a weak acid anion exchange resin in the hydroxyl form is used in the third colum. The regeneration of the first column is effected with the aid of an amonium hydroxide solution and softened raw water from the discharge of the second column so as to convert the weak base anion exchange resin into the hydroxyl form. The weak acid cation exchange resin in the second column is regenerated with diluted sulfuric acid and the weak base anion exchange resin in the third column is converted by the introduction of carbon dioxide, to a major portion in the bicarbonate form and to a minor portion in the hydroxyl form. Thereafter, the three columns are again ready for the next desalination process which, then, however, takes place in the opposite direction, i.e. the raw water is now charged into the third column.
According to Epstein et al, the three bed Desal process is not very effective and yields the poorest quality desalted water of the four processes described. There is also a possibility of calcium carbonate precipitation in the first colum. This, Epstein et al state, cannot be tolerated because it allegedly reduces the anion exchange resin capacity, consumes carbon dioxide, increases the costs for pumping and interferes with the flow conditions in the resin bed.
A second Desal process described by Epstein et al operates with a two-bed system. In this process, the third column of the three-bed Desal system is replaced by a decarbonator. The process is similar to the three bed process, with the exception that the effluent from the second column, which column contains the weak acid cation exchange resin, is conducted through the decarbonator in order to remove the carbon dioxide. The regeneration of the weak base and weak acid ion exchange resins is also similar to that described in connection with the three-bed process. However, after the weak base anion exchange resin has been regenerated to the free base form with an ammonium hydroxide solution and softened raw water, it must be converted to the bicarbonate form by means of a treatment with carbon dioxide. The longer time required for this conversion makes it necessary to use a larger quantity of resin and longer columns. Further, the two bed Desal process, as well as the three bed process, employs an ammonium hydroxide solution as a regenerating agent, and this is entirely unsuitable for some applications, particularly for the preparation of drinking water.
Epstein et al also disclose the so-called SUL-biSUL process which is performed in a two-bed system comprising a column with a strong acid resin in hydrogen form and a subsequent column with a strong base resin in sulfate form. After the raw water has been desalted, the cation exchange resin is loaded with cations, and the anion exchange resin is present in the chloride or bisulate form. The effluent from the anion exchange column in this process is freed of carbon dioxide in a decarbonator.
For regeneration, the cation exchange resin is treated with diluted sulfuric acid, and the anion exchange resin present in the chloride or bisulfate form, respectively, is treated with raw water so that the sulfate/bisulfate equilibrium is reversed until the resin has been returned to the sulfate form.
The main drawback of the SUL-biSUL process is the limitation of its use to waters which have a sulfate to chloride ratio of 9:1 or greater. Moreover, the capacities of both resins used in this process are relatively low, which necessitates the use of either long columns or frequent regenerations, which has an adverse effect. Also, the regeneration with raw water produces large amounts of waste water which contain sulfuric acid and must therefore be neutralized.
Finally, Epstein et al discuss the RDI process. This process is performed in a four-bed system comprising a first column with a strong base resin in bicarbonate form, a second column with a weak acid exchange resin in hydrogen form, a third column with a strong acid resin in hydrogen form and a fourth column with a weak base resin in free base form and, connected to the fourth column, a decarbonator. During the desalination, the first column is charged with chloride and sulfate ions and the second column retains the cations. The discharge from the second column contains carbonic acid which passes through the third and fourth columns and is removed in the decarbonator. Neutral salts which have passed through the first and the second columns are hydrolyzed in the third column and the resulting mineral acid is absorbed in the fourth column by the free base form of the weak base resin. For regeneration, sulfuric acid is flowed from the bottom to the top through the strong acid cation exchange column, and the resulting effluent is charged into the top of column two (the weak acid column) to regenerate the weak acid cation exchange resin. The charged strong base anion exchange resin is treated with sodium bicarbonate solution and the resulting effluent is used to regenerate the weak base anion exchange resin in column four by flowing the effluent from the bottom to the top through column four. The major drawback of this process lies in the high costs for the sodium bicarbonate.
In the above-mentioned full desalination processes which provide a combination of cation and anion exchange resins, the regeneration effect, when carbon dioxide is used for the regeneration, is poor and the ion exchange takes place very slowly.